1. Field of the Invention
This invention relates to polyester resins used in high solids coatings and more particularly relates to such a resin having a relatively low viscosity at high solids concentration. The invention further relates to the method of manufacturing such a resin.
2. History of the Prior Art
Low molecular weight polyesters have been widely used in high solids coatings as a binder which will give low volatile organic compounds (VOC) contents when crosslinked with melamine (or urea) formaldehyde resins or isocyanate functional resins. One of the impediments to getting lower volatiles with these polyesters by further reduction in molecular weight and less solvent (VOC) demand for viscosity reduction is their molecular weight distribution. This problem is discussed in U.S. Pat. No. 4,045,391. As very low molecular weights (500 to 1500 Wn) are approached, the random nature of the polyesterification reaction leaves proportionally larger amounts of the glycols, polyols and polyacids and their simple esters in the final reaction product. These have sufficient volatility to contribute to the VOCs when tested. Also the higher molecular weight portion of the distribution contributes much more heavily to the viscosity and resultant solvent demand of the polyester.
Such polyesters made from oxirane-anhydride single step bulk polymerizations are known in the prior art. Examples of patents describing such polymerizations are U.S. Pat. Nos. 3,376,272; 3,089,863; 2,779,783 and 3,374,208.
Narrowed molecular weight ranges have, however, been difficult to obtain due to random chain length formation in bulk polymerizations and since the temperatures usually associated with esterification reactions cause transesterification and equilibrium reactions.
It has been known that low molecular weight short chain products could be obtained by stepwise reaction of oxiranes and anhydrides with respectively carboxy or hydroxy terminated compounds. Most of such products usually have 20 or fewer combined carbon and oxygen atoms in a single main chain. Such products which have more than 20 combined carbon and oxygen atoms are still not as good as desired when used as prepolymers due to undesirable properties, e.g. an undesirably high percentage of volatile components, formation of soft polymers, formation of polymers having a poor combination of hardness and flexibility, or a viscosity lower than desired. In addition, undesirably large amounts of crosslinking agents may be required to form a suitable polymer from the prepolymer. Numerous patents describe such low molecular weight, short chain products, including U.S. Pat. Nos. 3,857,817 and 4,322,508.
U.S. Pat. No. 4,045,391 is directed toward the preparation of low viscosity polyesters containing fatty acid constituents having a narrowed molecular weight by certain stepwise anhydride-oxirane reactions. The compounds and methods of this patent are not, however, very desirable since the compounds require the presence of ester side chains which generally reduce weatherability and chemical resistance and increase color. In addition such side chains are commonly long and cause undesirable resin softness and may reduce reactivity do to hindrance.